Porcelain enamel products are widely used as kitchen and table-top articles, components of heating appliances and components of cooking appliances, combustion equipment, bathtubs, interior and exterior materials of buildings, and the like. Porcelain enamel products are generally produced by firing twice; a ground coat enamel is first provided on the steel sheet and fired, a cover coat enamel is further applied thereon, and fired again. In order to reduce the production cost, however, a production method of direct enameling for only once (“direct-on enameling”) comprising providing the cover coat enamel directly on the steel sheet followed by firing is employed. However, in the production method of direct-on enameling, it is necessary to perform pretreatment on the steel sheet before enameling, such as intense pickling and Ni dipping treatment. Further, as a steel sheet for obtaining favorable adhesion of the enamel products with the steel sheet in a direct-on enameling, a rolled sheet of high oxygen steel containing oxygen at a high concentration, which is produced by reducing C content in the steel-making stage and produced by continuous casting without performing deoxidation treatment, is widely utilized. However, a high oxygen steel generally suffers poor workability, and its application to usages requiring severe processing is limited.
As a means for improving the adhesion with the enamel in case of applying once, in Japanese Patent Publication No. 24413/1979 is proposed to perform, instead of the surface treatment of the conventionally employed nickel dipping method that is generally performed before enamel applying, covering the surface of the steel material made of the conventional carbon steel sheet with an alloy of one or two types of metals selected from Ni and Fe with one or two types of metals selected from Mo and W. However, there is no description on its workability.
On the other hand, in usages requiring severe workability such as kitchen wares or bathtubs, rolled sheets of Ti-added steel or B-added steel have been used. However, as is disclosed in Japanese Patent Laid-Open No. 140286/1998, although Ti-added steel or B-added steel is superior in workability, black specks defects generate in the enamel layer in case direct-on enameling is performed. Accordingly, the enamel layer had to be formed by means of ground coat enamel finishing or by applying the cover coat and ground coat enameling in two times.
As described above, there is required a steel sheet for porcelain enameling available by reduced production steps and energy consumption, thereby reduced in production cost, yet improved in workability and having an enamel layer with high adhesion even in case the cover coat enamel is applied only once.
The present invention has been made in the light of the circumstances described above, and the objects thereof are to provide steel sheet for porcelain enameling having excellent workability yet capable of providing an enamel layer having excellent adhesion with the steel sheet by applying direct-on enameling of cover coat enamel only once and still free of black specks defects, to provide the method for producing the same, as well as to provide a porcelain enamel product and the method for producing the same, in which low carbon Al-killed steel sheet, high oxygen steel sheet, Ti-added steel sheet, Nb-added steel sheet, Ti—Nb-added steel sheet or B-added steel sheet is used.